Answer:
The probability that the mean battery life would be greater than 533.2 minutes (in a sample of 75 batteries) is
Step-by-step explanation:
The main thing we have to take into account in this question is that we are about to find the probability of a <em>sample mean</em>. The distribution for <em>sample means</em> follows a <em>normal distribution</em> with mean
and standard deviation
. Mathematically
![\\ \overline{x} \sim N(\mu, \frac{\sigma}{\sqrt{n}})](https://tex.z-dn.net/?f=%20%5C%5C%20%5Coverline%7Bx%7D%20%5Csim%20N%28%5Cmu%2C%20%5Cfrac%7B%5Csigma%7D%7B%5Csqrt%7Bn%7D%7D%29)
For values of the sample
, no matter the distribution the data come from.
And the variable <em>z</em> follows a <em>standard normal distribution</em>, and, as we can remember, this distribution has a mean = 0 and a standard deviation = 1. Mathematically
[1]
That is
![\\ z \sim N(0, 1)](https://tex.z-dn.net/?f=%20%5C%5C%20z%20%5Csim%20N%280%2C%201%29)
We have a variance of 3364. That is, a <em>standard deviation</em> of
![\\ \sigma^2 = 3364; \sigma = \sqrt{3364} = 58](https://tex.z-dn.net/?f=%20%5C%5C%20%5Csigma%5E2%20%3D%203364%3B%20%5Csigma%20%3D%20%5Csqrt%7B3364%7D%20%3D%2058)
The population mean is
![\\ \mu = 530](https://tex.z-dn.net/?f=%20%5C%5C%20%5Cmu%20%3D%20530)
The sample size is ![\\ n = 75](https://tex.z-dn.net/?f=%20%5C%5C%20n%20%3D%2075)
The sample mean is ![\\ \overline{x} = 533.2](https://tex.z-dn.net/?f=%20%5C%5C%20%5Coverline%7Bx%7D%20%3D%20533.2)
With all this information, we can solve the question
The probability that the mean battery life would be greater than 533.2 minutes
Using equation [1]
![\\ z = \frac{\overline{x} - \mu}{\frac{\sigma}{\sqrt{n}}}](https://tex.z-dn.net/?f=%20%5C%5C%20z%20%3D%20%5Cfrac%7B%5Coverline%7Bx%7D%20-%20%5Cmu%7D%7B%5Cfrac%7B%5Csigma%7D%7B%5Csqrt%7Bn%7D%7D%7D)
![\\ z = \frac{533.2 - 530}{\frac{58}{\sqrt{75}}}](https://tex.z-dn.net/?f=%20%5C%5C%20z%20%3D%20%5Cfrac%7B533.2%20-%20530%7D%7B%5Cfrac%7B58%7D%7B%5Csqrt%7B75%7D%7D%7D)
![\\ z = \frac{3.2}{\frac{58}{8.66025}}](https://tex.z-dn.net/?f=%20%5C%5C%20z%20%3D%20%5Cfrac%7B3.2%7D%7B%5Cfrac%7B58%7D%7B8.66025%7D%7D)
![\\ z = \frac{3.2}{6.69726}](https://tex.z-dn.net/?f=%20%5C%5C%20z%20%3D%20%5Cfrac%7B3.2%7D%7B6.69726%7D)
![\\ z = 0.47780](https://tex.z-dn.net/?f=%20%5C%5C%20z%20%3D%200.47780)
With this value of z we can consult a <em>cumulative standard normal table</em> (or use some statistic program) to find the cumulative probability for <em>z</em> (and remember that this variable follows a standard normal distribution).
Most standard normal tables have values for z for only two decimals, so we can round the previous value for z as z = 0.48.
Then
![\\ P(z](https://tex.z-dn.net/?f=%20%5C%5C%20P%28z%3C0.48%29%20%3D%200.68439)
However, in the question we are asked for
. As well as all normal distributions, the standard normal distribution is symmetrical around the mean, and we have
![\\ P(z>0.48) = 1 - P(z](https://tex.z-dn.net/?f=%20%5C%5C%20P%28z%3E0.48%29%20%3D%201%20-%20P%28z%3C0.48%29)
Thus
![\\ P(z>0.48) = 1 - 0.68439](https://tex.z-dn.net/?f=%20%5C%5C%20P%28z%3E0.48%29%20%3D%201%20-%200.68439)
![\\ P(z>0.48) = 0.31561](https://tex.z-dn.net/?f=%20%5C%5C%20P%28z%3E0.48%29%20%3D%200.31561)
Rounding to four decimal places, we have
![\\ P(z>0.48) = 0.3156](https://tex.z-dn.net/?f=%20%5C%5C%20P%28z%3E0.48%29%20%3D%200.3156)
So, the probability that the mean battery life would be greater than 533.2 minutes is (in a sample of 75 batteries)
.